1. Field of the Invention
The invention relates to a fuel property determination apparatus for an internal combustion engine. More specifically, the invention relates to a fuel property determination apparatus for an internal combustion engine that makes it possible to more accurately determine, even at low temperatures, whether or not the fuel supplied to the internal combustion engine is heavy fuel.
2. Description of Related Art
In the relevant technical field, it is known that the property of the fuel supplied to the internal combustion engine has an influence on the control of an air-fuel ratio and the like in the internal combustion engine. For example, the volatility of heavy fuel is lower than the volatility of standard fuel. Therefore, when the control adapted for standard fuel is executed, the operability of the internal combustion engine and/or the exhaust emission properties may deteriorate. Thus, in the relevant technical field, various attempts to make a determination on the property of fuel have been made with a view to executing the control corresponding to the property of fuel.
For example, in an internal combustion engine that is equipped with an ignition timing controller that executes feedback control of an ignition timing such that the rotational speed of the internal combustion engine converges to a target rotational speed at the time of idling after startup, it is proposed to calculate an ignition timing correction amount at the time when the engine rotational speed stabilizes with respect to the target rotational speed and an ignition timing correction amount that converges after stabilization of the rotational speed, and to make a determination on the property of fuel based on a difference between those ignition timing correction amounts (e.g., see U.S. Pat. No. 3,863,362).
Besides, it is also known to make a more accurate determination on the property of fuel based on a ratio of a correction amount from a basic ignition timing corresponding to a target rotational speed (which will be referred to hereinafter as “an ignition timing correction amount” in some cases) to an allowable range of an ignition timing that is determined in accordance with the target rotational speed, a temperature and the like of an internal combustion engine (which will be referred to hereinafter as “a maximum ignition correction width” in some cases).
As described previously, in the relevant technical field, various attempts to make a determination on the property of fuel have been made with a view to executing the control corresponding to the property of fuel. It is also known to make a more accurate determination on the property of fuel based on the ratio of the ignition timing correction amount to the maximum ignition correction width. Thus, the air-fuel ratio and the like can be controlled in accordance with the property of fuel, and the operability of the internal combustion engine and/or the exhaust emission properties can be held good.
However, the rotational speed tends to greatly fluctuate immediately after startup of the internal combustion engine, for example, in a cold land. In this case, the ignition timing correction amount greatly fluctuates as well. Therefore, with a method of making a determination on the property of fuel according to the related art as described above, it may be erroneously determined that heavy fuel is used despite the use of standard fuel. This problem tends to manifest itself as the use of automobiles becomes widespread all over the world these days.